I don't take supps except for protein and I don't switch unless it is unavoidable.

Worst:

Bodytech Casein Protein Powder.

It didn't mix well no matter how much water of milk I used. It became gritty sludge and was disgusting. I purchased it only because it was on sale and I thought I'd try it. I was lucky that the store took it back, after I tried to use it for two days.